This invention relates to synthetic ester compositions and in particular to compositions based on a blend of poly(neopentyl) polyol ester and an ester of a polyol containing at least two hydroxyl groups, which is inhibited with a unique combination of additives. The compositions are particularly well suited for use as a coolant/lubricant in a rotary screw air compressor. The coolant/lubricant compositions minimize use of esters formed from highly odoriferous acids, yet provide extended life, higher temperature operation, excellent demulsibility, biodegradability, rust protection and hydrolytic stability--a combination of properties not available in existing synthetic compressor coolant/lubricant compositions.
It is well known to use hydrocarbon lubricating oils in rotary screw compressors. The oil seals the rotors, lubricates the bearings, cools the compressed gases and removes water condensed from the compressed gases. The high temperature and pressure and presence of water causes the hydrocarbon oils to break down and create sludge in a relatively short time.
Efforts to lengthen the useful life of coolants for air compressors have led to the use of synthetic esters as basestocks. Typical operating temperatures are between about 80.degree. to 104.degree. C. (170.degree. to 220.degree. F.). While these temperatures are not high for a synthetic lubricant, the environment is highly oxidative so that mineral oils tend to break down after about 1,000 hours of use and must be changed. Several of the synthetic coolant/lubricants presently in use extend the intervals between changing lubricants to as much as 8,000 hours.
A synthetic ester coolant based on diesters of adipic acid and phthalic acid have been used for more than 25 years in a variety of compressors including rotary screw compressors. Other synthetic coolants in use are based on synthetic hydrocarbons or poly alpha olefins (PAO's), polyalkylene glycols (PAG's), silicones, and mixtures of synthetic hydrocarbons and dibasic acid esters. All of these products provide extended lubricant life when compared to petroleum based coolants used in the highly oxidative environment in a screw compressor. However, none of the available lubricants are able to provide all of the desired properties, in particular the combination of biodegradability with hydrolytic stability, rust protection, and demulsibility while avoiding offensive odors.
In addition to these performance properties, certain physical properties are required, such as providing efficient cooling, low temperature start-up, sealing of the rotors, and lubrication of the bearings. In particular, the desirable physical properties include a viscosity at 100.degree. C. (212.degree. F.) of at least 8 cSt, a viscosity index of at least 140, a flash point of at least 260.degree. C. (500.degree. F.), and a pour point of at most -50.degree. C. (-58.degree. F.).
A wide operating temperature range for the coolant is highly desirable. In order to permit low temperature starting, a lubricant having a pour point less than -50.degree. C. (-58.degree. F.) is required. Suitable high temperature viscosity properties and low volatility are needed to permit suitable lubrication at higher operating temperatures. Thus, the coolant composition should have a flash point above 260.degree. C. which is required by some manufacturers for safety reasons.
All screw compressor coolant/lubricants will come into contact with water condensed from the compressed air. Therefore, any lubricant must exhibit good hydrolytic stability and provide good rust and corrosion protection. It is an added advantage to provide a lubricant exhibiting good demulsibility. This will permit separation of water from the spent lubricant and facilitate disposal and recycling of the spent lubricant.
Increased biodegradability is also highly desirable. In addition, the elimination of heavy metals, utilized in some commercial coolants, Is also desirable from an environmental point of view.
It is also highly desirable to avoid objectionable odors. Thus, it becomes desirable to avoid or minimize the use of esters formed from carboxylic acids having less than seven carbon atoms. Finally, adequate regulatory listing is desirable.
Existing petroleum based and synthetic air compressor coolant/lubricants are not capable of providing all of the desired performance and physical properties for modern rotary screw compressors. Often, modification of one component of a synthetic lubricant will improve a certain desired property, but at the expense of another. For example, coolants based on PAO's have poor biodegradability; coolants based on adipate diesters cannot provide the viscosity, or flash point properties; coolants based on phthalate diester have poor biodegradability and viscosity indices; coolants based on silicones have poor biodegradability and rust protection; coolants based on polyalkylene glycol have poor demulsibility and inadequate rust protection; and coolants based on mixtures of PAO's and diester do not provide the desired biodegradability.
The following table summarizes the desired properties generally available from current types of rotary screw compressor coolant/lubricants. While several different types may be noted as having satisfactory performance of a particular property, there will be variations within acceptable ranges, some of which may be significant to the user. An "X" indicates that the referenced type of commercially available coolant/lubricant can provide the desired property, and an "O" indicates that it is not fully acceptable.
TABLE 1 __________________________________________________________________________ SIGNIFICANT DESIRED PROPERTIES FOR ROTARY SCREW COMPRESSOR COOLANT/LUBRICANT COMPOSITIONS PAO PAG Desired* Mineral (Polyalpha (PolyAlkylene Desired Properties Composition Oil Olefin) Diester PAO/Ester Glycol) Silicone Phosphate PAG/Ester __________________________________________________________________________ Pour Point &lt; -50.degree. C. x .smallcircle. x .smallcircle. x x x .smallcircle. x Flash Point &gt; 260.degree. C. x .smallcircle. .smallcircle. x .smallcircle. x x x x Good Rust Prevention x x x x x .smallcircle. .smallcircle. .smallcircle. .smallcircle. Good Demulsibility x x x x x .smallcircle. x .smallcircle. .smallcircle. Good Hydrolytic Stability x x x .smallcircle. x x x .smallcircle. x Biodegradable x .smallcircle. .smallcircle. .smallcircle. .smallcircle. .smallcircle. .smallcircle. .smallcircle. .smallcircle. Viscosity Index &gt; 140 x .smallcircle. x .smallcircle. x x x .smallcircle. x __________________________________________________________________________ *The poly(neopentyl polyol) ester based compositions prepared in accordance with the invention exhibit all of the desired properties.
One of the most widely used high performance rotary screw compressor coolants is a PAG/ester synthetic lubricant of the type described in U.S. Pat. No. 4,302,343 to Carswell et al. The Carswell PAG/ester lubricant is an inhibited blend of a polyalkylene glycol with hindered alkanoic ester of aliphatic polyhydric alcohols having three to eight hydroxyl groups and five to ten carbon atoms. When properly blended and mixed with additives, the resulting synthetic coolant/lubricant has been found to satisfy the high temperature viscosity requirements and exhibit stability to the heat, air and water environment. However, ferrous metal corrosion resistance and demulsibility has been found less than satisfactory. The presence of the heavy metal barium also presents disposal concerns.
U.S. Pat. No. 4,175,045 to Timony describes a compressor lubricant formed of a polyol ester of a carboxylic acid having from about four to thirteen carbon atoms. The polyols utilized are pentaerythritol, dipentaerythritol, trimethylolpropane, or combinations thereof. The lubricants are blends of the pentaerythritol ester and dipentaerythritol esters. While providing satisfactory useful life and temperature characteristics, the viscosity at 100.degree. C. (212.degree. F.) and pour point do not meet the desired values as set forth in Table I. In addition, the lubricants are based on substantial amounts of esters formed from valeric acid and thus in use present undesirable odor problems. Improvements in rust prevention and demulsibility are also needed.
As noted, the prior art describes a wide variety of synthetic lubricants based on various polyol esters and blends thereof admixed with various additives to improve performance. While these synthetic rotary screw air compressor coolant/lubricants are presently in wide use, it remains desirable to provide an improved coolant/lubricant composition which can provide all the significant desired properties noted in Table I, particularly increased oxidative stability, improved demulsibility, improved rust protection, increased biodegradability and low odor in use.